In 1940 the Vichy government of French Indo-China was isolated and threatened by the imperialist Japanese, the neighbouring Thais and by native rebel movements. The French had about 50,000 colonial and metropolitan troops stationed in the colony. They outnumbered the small French civilian population of 40,000 colonists in a territory of 25 million Indo-Chinese. The French collapse in the spring of 1940 resulted in the German occupation of 60% of France, but Marshall Pétain’s Vichy government retained control of the remainder, as well as France’s colonial empire. Indo-China was, however, cut off from re-supply from Vichy France. A British blockade proved effective, meaning that troops could not be rotated for the duration of the war, nor could parts be obtained for military equipment. Fuel supplies could also not be replenished so long as the petroleum-short Japanese Empire controlled the Asian theatre.

Vichy diplomats attempted to persuade Germany to allow them to ship arms and equipment to Indo-China, appealing to the Germans on racial grounds, pointing out the possibility of the ‘white race’ losing ground in Asia. The Germans would promise only to speak to the Japanese. At the same time Vichy was fending off offers from the Chinese to occupy Indo-China to ‘protect’ it from the Japanese. Aware of China’s own irredentist claims in the area, the French doubted they would ever get their colony back if the Chinese were allowed in.

The Japanese deliver a shock

As France fell, the Japanese began to make demands of the Governor-General of Indo-China, General Catroux. When the General acceded to demands that rail traffic to China be stopped he was promptly replaced. Vichy named the loyal commander of the FNEO (Forces Navales d’Extreme-Orient), Vice-Admiral Jean Decoux, as Governor General. By September Decoux was facing far greater demands from the Japanese, including the right to station and transport troops through Indo-China, the use of selected airfields, and the evacuation of a hard-pressed Japanese division fighting in China through the port of Haiphong. An appeal to the Americans for help was poorly received.

Aware of his predecessor’s fate, Decoux hesitated, signing the agreement just before the Japanese ultimatum ran out. The Japanese division was tired of waiting, however, and proceeded to cross the border on September 22, 1940, attacking the Tonkinese cities of Dong Dang and Lang Son with tanks and infantry. The Japanese navy made landings along the coast, Haiphong was bombed, and the Japanese Air Force flew repeatedly over Hanoi. The Japanese offensive came as a shock to some senior French officers, who still believed in natural European superiority and often talked about taking tough action against the Japanese. Dong Dang fell immediately, and Lang Son fell two days later, with many of the locally raised colonial units breaking and running before their first experience of artillery and disciplined infantry attacks carried out by veteran soldiers. French intelligence had reported that the Japanese were demoralized, but it was the French who collapsed under pressure. Local villagers revealed French positions to the Japanese, French artillery fired on French positions, ammunition ran out quickly, and over a thousand Indo-Chinese troops deserted.
A statement issued by the Japanese emperor on October 5 called the Lang Son attack unfortunate but not important. The French prisoners were released, but 200 German legionnaires who had been separated from the other French prisoners were not released until the 13th of October. The pursuing Chinese army made numerous forays across the frontier, and the French administration remained fearful of a full-scale Chinese invasion until the end of the war. The French had lost 800 men in two days of battle with the Japanese.

Nationalist rebellions

The fall of Lang Son had almost immediate consequences for French rule. Discontented locals had witnessed how easily an Asian army defeated the whites. Vietnamese nationalist Tran Trung Lap was able to raise some 3,000 men in the Lang Son region, many of them deserters from the Indo-Chinese units defeated by the Japanese. Their arms were provided from French stocks captured by the Japanese. The returning French demonstrated they could still deal with a poorly trained rabble, and quickly drove the revolutionaries into the mountains, where planes and artillery hammered them. Tran Trung Lap was ambushed, and though he escaped the massacre of his men by machine-gun, he was shortly after captured and executed at Lang Son in December.

In the south of Vietnam, then known as Cochin China, an even more dangerous rebellion broke out in late November. Thai troops had begun to deploy along the Cambodian border and most of the garrisons in Cochin China had been sent to the frontier. Fighting broke out in the My Tho region and French police found themselves overwhelmed. The rebellion spread to Saigon and a number of southern provinces. A battalion of the Foreign Legion and a battalion of Tonkinese colonial troops on their way to Cambodia were diverted to the south and, with the help of artillery, air and naval detachments, quickly repressed the rebellion with utmost ruthlessness. The French had made their point, and could now send their forces west to deal with the Thais.

War with Thailand

The French now had to deal with a growth of militarism and Thai nationalism in neighbouring Thailand (the name was changed from Siam in 1938). Just as Germany sought to regain the territories lost in the Treaty of Versailles, Thailand was eager to retake the ethnic Thai lands along the Mekong River it was forced to cede to the French colony of Laos in 1904. In 1907 the French had also forced Siam to cede the largely Khmer provinces of Siemreap, Sisophon and Battambang to French Cambodia. The pro-Japanese government of Marshal Pibul Songgram sensed an exploitable weakness in the now isolated French colony, and began a military campaign to retake these territories after the French rejected demands for their return in October 1940.

The Thais had signed a non-aggression pact with the French in June 1940, but failed to ratify it after the collapse of metropolitan France. By October Marshal Songgram had mobilized 50,000 troops (in five divisions) and obtained 100 modern fighters, bombers and seaplanes from Japan. The Thai air-force was now three times the size of that available to the French, with the new aircraft added to the 100 American planes obtained between 1936 and 1938 (mostly Vough Corsairs and Curtiss Hawks). The Thai navy had also been equipped with modern ships and outclassed the French colonial fleet on paper at least. Border skirmishes began in November and the Thais crossed the Mekong in December. Hard-pressed elsewhere, the French could only commit fourteen battalions to the defence of Battambang Province.

On January 5, 1941, the Thais launched a full attack with artillery and aerial bombardment of French positions. The Thai offensive covered four fronts:

1) North Laos, where the Thais took the disputed territories with little opposition
2) South Laos, where the Thais crossed the Mekong by the 19th of January
3) The Dangreks Sector, where confused fighting went back and forth
4) Colonial Route 1 (RC 1) in Battambang province, where the heaviest fighting occurred.

The initial advance on the RC 1 was repulsed by the Cambodian Tirailleurs (riflemen). The main Thai column ran into a French counter-attack on January 16, colliding with the French at Yang Dam Koum in Battambang. The Thai force was equipped with Vickers 6-ton tanks while the French lacked any armour. The French counter-offensive had three parts:

1) A counter-attack on the RC 1 in the region of Yang Dam Koum
2) An assault by the Brigade d’Annam-Laos on the islands of the Mekong River
3) Operations by the naval ‘Groupement occasionnel’ against the Thai fleet in the Gulf of Siam

The main thrust of the offensive was by Col. Jacomy’s forces along the RC 1. The attack at Yang Dam Koum was a debacle from the start. The assault forces consisted of one battalion of Colonial Infantry (European) and two battalions of ‘Mixed Infantry’ (European and Indo-Chinese). The forest made artillery operations difficult, French aircraft never showed, leaving the skies to the Thai air-force, and radio communications were poor. The French transmitted orders using Morse code, perhaps explaining why the Thais often anticipated their movements. A complete rout was prevented when the Thais ran into a battalion of the Fifth regiment of Legion infantry at Phum Préau. The legionnaires were hit hard by a Thai armoured assault, but brought up two 25mm and one 75mm gun for use against the tanks. The motorized detachment of the 11th Regiment of Colonial Infantry reinforced the line, and three Thai tanks were destroyed, the rest deciding to retire. The diversionary assault on the Mekong was successful, but the largest battle of the war was to be fought in the Gulf of Siam.

Naval war in the Gulf of Siam

The French navy was all important in Indo-China, as with any overseas colony. The modest force had a virtually non-existent role in the great Asian war of 1941-45, being unable to resist either Japanese advances or Allied blockades, but they were nevertheless to have one great, unexpected battle before meeting an ignominious end. The fleet in Indo-China was divided into two parts with separate levels of responsibility. The FNEO was assigned responsibility for the overall defence of French colonies in Indo-China and the Pacific, while the Marine Indochine with its river gunboats was responsible for interior security in Indo-China.

With the land war going badly for the French, it was decided to send the small French fleet to the Gulf of Siam to engage a Thai naval force supporting the flank of the Thai advance. The Thai ships had been spotted lying at anchorage in the Koh Chang islands by a French navy flying boat. The French task-force (or Groupement occasionel) consisted of the light cruiser Lamotte-Piquet, the two colonial sloops Dumont d’Urville and Amiral Charner, and the WW1 vintage gunboats Tahure and Marne.

During the night of January 16 the French ships closed in on the islands, dividing themselves into three groups to cover the exits from the island group. On the morning of the 17th the French roared in under cover of the mist to engage the Thais. The Thai ships included three Italian-built torpedo boats and the dual-pride of the Thai fleet, the two new Japanese-made armoured coastal defence ships with 6” guns, Donburi and Ahidéa. The French were surprised to find both coastal defence ships there, as they expected only the Ahidéa, but the Donburi had arrived the day before in a standard rotation. The French lost the advantage of surprise when an overeager Loire 130 seaplane tried to bomb the Thai ships. The Thais received the French with the opening salvoes of the battle at 6:14 AM. The Lamotte-Piquet quickly inflicted fatal damage on the Ahidéa with gunfire and torpedoes, forcing it to run aground. By 7 AM French guns had sunk all three torpedo boats.

The Donburi was spotted attempting to escape through the 200m high islands and the French cruiser set off in pursuit. The Donburi was set afire but continued to engage the cruiser and the sloops, which now began to pour fire into the Donburi. Badly damaged and listing to starboard, the Donburi eventually disappeared behind an island and the French broke off. Later in the day the Donburi was taken in tow by a Thai transport but capsized soon after. Throughout the engagement the French sailors were impressed by the courage of the Thai sailors under fire.

The French ships were unable to exploit their victory, however, due to the arrival of Thai Corsairs targeting the Lamotte-Piquet . Fierce anti-aircraft fire drove off the attacks, and by 9:40 AM the French turned for home. In a brief but decisive engagement the Thai fleet had been destroyed at negligible cost to the French. It appeared at the time to be a sudden and dramatic reversal of French fortunes.

Aftermath

The Japanese had seen enough and accompanied an offer to mediate the conflict with the arrival of a powerful naval force off the mouth of the Mekong River to encourage negotiations. A tentative armistice was imposed on January 28, but Thai provocations on the frontier continued until a formal armistice was signed aboard the Japanese battleship Natori off Saigon. The extent of Thai-Japanese collaboration was revealed when a Japanese-imposed treaty between Vichy and Thailand was signed on May 9, 1941. The disputed territories of Laos, part of the Cambodian province of Siem Réap and the whole of Battambang were awarded to Thailand. The conflict had cost the French over 300 men and a further loss of prestige amongst its colonial subjects. European troops and material losses could not be replaced due to the blockade. The French garrison remained highly demoralized until the Japanese coup in 1945 destroyed the Vichy colonial army in Indo-China.

In the end the Thais fared little better. The Khmers largely evacuated the lost Cambodian territories, preferring French rule, and Thailand itself was soon occupied by its more powerful ally, the Japanese. American Flying Fortresses bombed Bangkok in 1942. The Thais declared war against the allies in 1944, but there was some confusion over whether the declaration was actually delivered to the US government, and after the war the Thai government certified the declaration of war as null and void. The uncomfortable affair was mutually forgotten. The disputed territories in Laos and Cambodia were returned to the new Gaullist government at the end of the war.

The French light cruiser Lamotte-Piquet was laid up shortly after the battle of Koh Chang due to the shortage of fuel. In 1945 the ship was bombed by American planes before being scuttled during the brutal Japanese coup of March 1945. The remaining naval force continued to escort convoys up and down the Vietnamese coast as best they could from 1941 to 1945. In their sudden seizure of Indo-China, the Japanese sank a number of French ships with shore fire, while the remainder were scuttled by their crews, who were then imprisoned. The French colonial armed forces in Indo-China had ceased to exist by the time the British and Chinese armies arrived after the Japanese surrender. It was the British and Chinese, rather than the men of Vichy, who would turn over the colony to Gaullist France at the end of World War II.

Here's how it was described in news reports from 1940, according to the Time magazine capsule.

Sept. 16, 1940

Prize to Nippon

Since Japan took advantage of the French collapse to impose "inspectors" French Indo-China and prevent munitions shipments to China, Tokyo has assumed that France's prize Asiatic colony was safe in Nippon's bag. But before Japan could actually move in and take, Oriental propriety demanded a righteous motive. It came last week when ousted Governor General Georges Catroux led important units of the Indo-Chinese colonial forces to the standard of General Charles de Gaulle, self-styled Commander of Free Frenchmen.
"A matter of grave moment to Japan," growled the Tokyo press, describing De Gaulle as a "mere puppet of the British government. Major General Issaku Nishihara, head of a big Japanese mission now in Indo-China to squeeze concessions out of the new Vichy-appointed Governor General Admiral jean Decoux, whipped out an ultimatum. He demanded on threat of invasion the use of the French Indo-China's chief port, Haiphong as a naval and air base, and permission to transport Japanese equipment and troops over the French-owned Indo-Chinese railway for an attack on South China.
Aware that the invasion of Yunnan Province through Indo-China would put a serious crimp in his resistance and enable the Japanese to cut the Burma Road, Generalissimo Chiang Kai Shek announced that Chinese troops would counter invade if Japanese forces were permitted to enter French Indo-China "under whatever pretext and whatever conditions."
Faced with a prospect of a double invasion in addition to internal revolt, Vichy's unhappy emissary pleaded for time to communicate with his Government. The French colonial government closed the port of Haiphong, suspended railway traffic throughout the colony, prepared to evacuate women and children to coastal towns. A Japanese fleet steamed outside Haiphong, and Japanese troops on the Japanese-occupied island of Hainan prepared for active duty.

Oct. 7, 1940
Gentlemen's agreement
At 4:30 one steaming afternoon last week in Hanoi, Governor General Admiral Jean Decoux of Indo-China and Japan's supreme penetrator General Issaku Nishihara sat down and signed an agreement. It permitted Japan to establish three air bases in Tonkin, the northern province of Indo-China, and to garrison the bases with 6,000 troops. The French out-Japanesed the Japanese in their comments. Admiral Decoux called the agreement "one of the greatest marks of confidence one country can give another." In Vichy, it was called a "gentleman's agreement." Five and one-half hours later, the friendly gentlemen of Japan went to work killing the confident gentlemen of France.
At 10 p.m. Japan's South China Army gave the French garrison at Dong Dang notice that they were moving in. The French decided to resist. In a two-hour skirmish, the French suffered about 100 casualties. Tokyo newspapers hailed the "peaceful penetration." French authorities put aside the honey and brought on the acid: "Anyone coming across the border in the middle of the night in combat formation and using arms is hardly friendly."
The timing of the Japanese drive was scarcely accidental. It was clear that the attack was very much in line with Axis grand strategy. If the downfall of of the British Empire was to be accomplished by control of the Atlantic-Pacific seaways at Gibraltar, Suez and Singapore, it was the job of the Japanese to capture Singapore. And the penetration of Tonkin was the first move in consolidation of the flanks preceding an attack on Singapore.

More than a war this was more like a single battle, but things happened. It shows one could not trust the Japanese those days. On the other hand this was just a formality because the colonial govenrnment neither had men, nor weapons to resist, so not accepting an agreement would have had the same result with even more atrocities. Remember there was no example of how the Japanese waged modern war before Pearl Harbor. Even in the Philipines pows paid the hard price. Thank you for remembering these hundred men who were massacred by the Japanese.

French forces in Indochina consisted of an army of approximately fifty thousand men, of whom twelve thousand were French, organised into forty-one infantry battalions, two artillery regiments, and a battalion of engineers.[1] The most obvious deficiency of the French army lay in its shortage of armour: it could only field twenty antiquated Renault FT-17s against the Thai Army's 134.
The Armée de l'Air had in its inventory approximately a hundred aircraft, of which around sixty could be considered first line. These consisted of thirty Potez 25 TOEs, four Farman 221s, six Potez 542s, nine Morane-Saulnier M.S.406, and eight Loire 130 flying boats.[2]
The Thai Army was a relatively well-equipped force. Consisting of some sixty thousand men,[3] it was made up of four armies, the largest of which was the Burapha Army with its five divisions. Independent formations under the direct control of the army high command included two motorised cavalry battalions, one artillery battalion, one signals battalion, one engineer battalion and one armoured regiment. The artillery had available a mixture of aged Krupp and modern Bofors howitzers and field guns, while sixty Carden Loyd tankettes and thirty Vickers six-ton medium tanks made up the bulk of the army's tank arm.
The Royal Thai Navy - consisting of several vessels, including two coastal defence ships, twelve torpedo boats and four submarines - was inferior to the French naval forces, but the Royal Thai Air Force held both a quantitative and qualitative edge over l'Armee de l'Air.[4] Among the 140 aircraft that composed the air force's first-line strength were twenty-four Mitsubishi Ki-30 light bombers, nine Mitsubishi Ki-21 medium bombers, twenty-five Hawk 75Ns pursuit planes, six Martin B-10 medium bombers, and seventy O2U Corsair light bombers.[5]

While nationalistic demonstrations and anti-French rallies were held in Bangkok, border skirmishes erupted along the Mekong frontier. The superior Royal Thai Air Force conducted daytime bombing runs over Vientiane, Sisophon, and Battambang with impunity. The French retaliated with their own planes, but the damage caused was less than equal. The activities of the Thai air force, particularly in the field of dive-bombing,[6] was such that Admiral Jean Decoux, the governor of French Indochina, grudgingly remarked that the Thai planes seemed to have been flown by men with plenty of war experience.[7]
In early January 1941, the Thai Burapha and Isan Armies launched their offensive on Laos and Cambodia. French resistance was instantaneous, but many units were simply swept along by the better-equipped Thai forces. The Thais swiftly took Laos, but Cambodia proved a much harder nut to crack.
At dawn on January 16, 1941 the French launched a large counterattack on the Thai-held villages of Yang Dang Khum and Phum Preav, initiating the fiercest battle of the war. Because of over-complicated orders and nonexistent intelligence, the French counterattacks were cut to pieces and fighting ended with a French withdrawal from the area. The Thais were unable to pursue the retreating French, as their forward tanks were kept in check by the gunnery of French Foreign Legion artillerists.
As the situation on land was exacerbating for the French, Admiral Decoux ordered the available French naval forces into action in the Gulf of Thailand. In the early morning of January 17, the French navy caught a Thai naval detachment anchored off the island of Ko Chang. The subsequent Battle of Ko Chang proved a victory for the French and resulted in the sinking of two Thai torpedo boats and a coastal defence ship.
On January 24, the final air battle took place when Thai bombers raided the French airfield at Angkor near Siem Reap. The last Thai mission commenced at 0710 hours on January 28, when the Martins of the 50th Bomber Squadron set out on a raid on Sisophon, escorted by three Hawk 75Ns of the 60th Fighter Squadron.[8][9]
The Japanese mediated the conflict, and a general armistice was arranged to go into effect at 1000 hours on January 28. On May 9 a peace treaty was signed in Tokyo,[10][11] with the French being coerced by the Japanese into relinquishing their hold on the disputed territories.

The resolution of the conflict was received with wide acclaim among the Thai people and was looked upon as a personal triumph for Phibun. For the first time, Thailand had been able to extract concessions from a European power, albeit a weakened one. For the French in Indochina, the conflict was a bitter reminder of their isolation following the Fall of France. In the French view, an ambitious neighbour had taken advantage of a distant colony cut off from her weakened parent. Without hope of reinforcements, the French had little chance of offering a sustained resistance.
However, the real beneficiaries of the conflict between Thailand and the Vichy French colony were the Japanese. They were able to expand their influence in both Thailand and Indochina.
The Japanese won from Phibun a secret verbal promise to support them in an attack on Malaya and Burma. [12] However, the Thai Prime Minister was fickle and he was quite ready to forget this promise if circumstances changed. His government also asked both the British and Americans for guarantees of effective support if Thailand were invaded by Japan.
On 8 December1941, the Japanese invaded Thailand at the same time they invaded Malaya.
After the war, in October1946, western Cambodia and the two Lao enclaves were only returned to French sovereignty after the French provisional government threatened to veto Thailand's membership in the United Nations. [13]

The French army suffered a total of 321 casualties, of whom 15 were officers. The total number of men missing after January 28 was 178 (6 officers, 14 non-commissioned officers, and 158 enlisted men).[14] The Thais had captured 222 men (17 North Africans, 80 Frenchmen, and 125 Indochinese).[15]
The Thai army suffered a total of 54 men killed in action and 307 wounded.[16] 41 sailors and marines of the Thai navy were killed, and 67 wounded. At the Battle of Ko Chang, 36 men were killed, of whom 20 belonged to HTMS Thonburi, 14 to HTMS Songkhla, and 2 to HTMS Chonburi. The Thai air force lost 13 men. The number of Thai military personnel captured by the French amounted to just 21.
About 30% of the French aircraft were rendered unserviceable by the end of the war, some as a result of minor damage sustained in air raids that remained unrepaired.[17] The Armée de l'Air admitted the loss of one Farman F221 and two Morane M.S.406s destroyed on the ground, but in reality its losses were greater.[18]
In the course of its first experience of combat, the Royal Thai Air Force claimed to have shot down five French aircraft and destroyed seventeen on the ground, for the loss of three of its own in the air and another five to ten destroyed in French air raids on Thai airfields.

Even though I already knew about this affair since a while ago, there´s one thing that I still don´t get:

What was the stand of Indochina when the Nip stepped in after the war with Thailand? To know this would clarify the picture a lot regarding what happened later.

Several things point out the confusion I´m writing about:

1. With Catroux already taken care of for proclaiming himself De Gaulle´s (according to Time), you might think that Decoux as his succesor was Vichy, taking into consideration how he asked for instructions when faced with the threat of double invasion, and that he was taking the colony with him in that direction.

2. However, there shouldn´t have been a perceived threat of invasion from the Nip - making it a single, not a double one - since with Vichy a German puppet, there should have been an at least theoretical alliance between France and Japan as members of the Axis.

3. On the other hand, there shouldn´t have been a similar perception towards China, if Indochina was leaning towards the Cross of Lorraine on the Allied side.

Was it just that in those dark days, seems to me that even murkier than those after Pearl Harbor, frenchmen in Indochina just couldn´t understand who was on whose side, with the american refusal when asked for help, the wounds from Mers-el-Kebir, fueled by Vichy propaganda, still too open, and with the doubt about the actions taken by their own government? Things weren´t as clean cut as they got afterwards, and after trying to make some sense out of points 1, 2 and 3, you might think that the colony was just trying to handle itself and stay as safe as possible in the middle of the cauldron.

Battleaxe​

Pic shows an attack on Sittang Bridge by the Duke of Wellington´s Regiment, during the ´42 retreat​

By David Manley
Despite the fall of France in 1940 the Vichy government still held sway over France's colonial territories abroad. One of these territories was French Indo-China, what we know today as Vietnam and Kampuchea (Cambodia). With the defeat of mainland France obvious doubt was thrown over her ability to defend the far flung outposts of the empire. French Indo-China held a particular attraction to her neighbour to the West, Thailand. In late 1940 the Thais began a series of cross border attacks by troops and aircraft against the French possessions, aimed at annexing those parts of French Indo China which the French were unable to hold on to. These attacks tended to concentrate in the coastal regions of Cambodia, to the west of Saigon. The Thais naturally saw themselves as filling a power vacuum which had been created by deflating French colonial interests. The French, however, had other ideas and a state of war was assumed between Vichy France and Thailand (then still often referred to as Siam). Since the main thrust of the Thai assaults had been along the coast the French considered a naval operation against the invaders, although there was grave concern over strengths of the Thai armed forces which would oppose such a mission.
The Royal Thai Navy could in no way be regarded as a pushover. A number of new vessels had recently been delivered from Japan and Italy. The major units of the fleet are listed in Table 1, and included two Japanese-built armoured coast defence vessels which displaced 2,500 tons and carried 8" guns, two older British built armoured gunboats with 6" guns, twelve torpedo boats and four submarines. In addition the Royal Thai Air Force had in its inventory over 140 aircraft, including relatively modem Mitsubishi Ki-30 light bombers, which saw extensive service against the French. These aircraft in themselves were quite capable of causing severe damage to any French naval mission which may be mounted. Other less capable aircraft in the Thai inventory included 25 Curtiss Hawk 75N, 70 Vought Corsair biplanes, 9 B-10 bombers and several Avro 504 trainers.
Despite the strengths of the Thai forces the French Governor General of Indo-China and Commander-in-Chief Naval Forces, Admiral Jean Decoux, decided that the naval mission should go ahead. A small squadron, the Groupe Occasionnel, was formed on 9th December 1940 at Cam Ranh Bay, near Saigon, under the command of Captain de Vaisseau Berenger. The squadron consisted of the light cruiser Lamotte-Picquet, the colonial sloops Dumont d'Urville and Amiral Charner, and the older sloops Tahure and Marne. There was no air cover to speak of, apart from eight Loire 130 seaplanes based at Ream which provided reconnaissance. Additional scouting was provided by three coastal survey craft, and intelligence gleaned from the local fishermen.
Berenger's squadron began training manoeuvres in Cam Ranh Bay shortly after coming together. Early in the new year, on January 13th 1941, Admiral Decoux formally requested Berenger to send the squadron against the Thais to act in support of a land offensive planned for the 16th. This operation was intended to throw back Thai forces which had been advancing along the coast. Because of the disparate speeds of the French ships Berenger sent the slower sloops on ahead, whilst he remained in Saigon to complete the final elements of the plan. Several options were currently being prepared, the Admiralty in Paris having recently given its formal blessing to the use of naval forces in support of the army. The final planning meeting of the 13th saw an immediate delay in the execution of the operation for 24 hours. With the plans finalised, Berenger sailed in Lamotte-Picquet, the delay in the start of the operation allowing him to refuel at Cap St. Jacques before the rendezvous with the slower ships at 16:00 on the 15th, 20 miles North of Poulo Condore.
The orders from Admiral Decoux were clear and simple, "Seek out and destroy the Siamese naval forces from Satahib to the Cambodian frontier". On the evening of the 15th, following a final conference on board the flagship, the squadron weighed anchor at 21 15 and closed the Thai coast at 14 knots, the best speed of the sloops. The French ships remained undetected as they entered the Gulf of Siam, but their quarry was not so lucky. The Loire 130s from Ream had completed a sweep of the coast from Krat to Satahib. They had located one coast defence ship and three torpedo boats at Koh Chang, and one gunboat, four torpedo boats and two submarines at Satahib. Their report was sent to Marine Headquarters in Saigon, who retransmitted the report to the Lamotte-Picquet. Berenger considered his options in the light of this intelligence and opted for a dawn attack on Koh Chang. He discounted an attack on Satahib because it was not possible for the sloops to reach the port until later in the day, by which time the Thai force was likely to have been alerted to the French presence and the element of surprise would be lost. In addition there was doubt as to the contribution which the harbour defences at Satahib could make. Finally the force at Koh Chang, although formidable, was the weaker of the two and was thought to offer the best chance for victory.
Berenger's plan of attack was as follows. The squadron would approach at dawn from the South West. Because the anchorage at Koh Chang was surrounded by islands and islets, many of which were over 200 metres high, the squadron would break up and use the cover of the islands to concentrate fire on portions of the Thai squadron whilst covering all the avenues of escape. The easternmost channel was regarded as the most likely route by which a breakout would be made - this was the most suitable route and was also the area in which the recce report had placed the largest Thai ships. The Lamotte-Picquet would head to the eastern side of the anchorage to block this route whilst the colonial sloops blocked the centre and pounded the Thai ships there. The smaller French ships would concentrate to the West.

[SIZE=+1]The Attack[/SIZE]
The French squadron closed on the anchorage at 05:30 on the 17th. At 05:45 they split into the three groups as planned, the Lamotte-Picquet heading for the Eastern part of the anchorage, Dumont d'Urville and Amiral Charner continuing to the central position and the Tahure and Marne heading for the Western side. Conditions were perfect - the weather was fine, the seas calm and almost flat. Sunrise was due at 06:30, and the scene was lit only by the first rays of light on the horizon and by the dim moonlight. A final aerial reconnaissance of the target area had been arranged using one of the Ream-based Loire 130s (Lamotte-Picquet carried two such aircraft, but these could not be launched due to catapult problems). At 06:05 the Loire 130 overflew the anchorage and reported two coastal defence ships. This came as a nasty surprise to the French - previous reports led them to believe that only one of the large defence ships was present, but during the night the Dhonburi had arrived to relieve the Sri Ayuthia, which was to return to Satahib later that day. Once their presence had been passed to the Lamotte-Picquet the aircraft attempted an attack of its own using bombs, but was forced off by a heavy barrage of AA fire. The effect of this mission was double edged - the French were now aware that they faced both the Thai heavy units, but the element of surprise had been wasted and there was still thirty minutes to go until sunrise. Caught napping by the oncoming French the Thais desperately began to raise steam and make preparations for slipping their anchors. The coastal defence ships had an advantage over their smaller consorts since, being diesel powered they were able to get under way almost immediately.
Despite their initial shock at coming under attack it was the Thai forces which opened fire first at 06:14 when the range had come down to 9,000 metres. The French responded almost immediately. Lamotte-Picquet fired at the dark shapes in the anchorage and fired a spread of three torpedoes at 06 20. One torpedo was seen to hit the coastal defence ship Sri Ayuthia. Heavily damaged by the torpedo and gunfire she headed for the cover of the islands and was finally beached on the mainland. The torpedo boat Trad then became the Lamotte-Picquet's next target. She was heavily damaged and also disappeared behind the cover of the islands, where she later sank. Fire then switched to the remaining torpedo boats, Chonburi and Songhkli, which were rapidly overwhelmed and abandoned. Gunfire from the sloops finished them off at 07:00.
At 06:38 the lookouts on the Lamotte-Picquet spotted the second coastal defence ship, the Dhonburi, at a range of 10,000 meters heading to the North West. A running battle ensued with the fire of both ships frequently blocked by the towering islets. The fire from the Thai ship was heavy, but inaccurate. By 07:15 fires could be seen on the Dhonburi, which then found herself engaged not only by the cruiser but also by the sloops. Believing they had a better chance of hurting the smaller French ships the Thais shifted their fire onto the Amiral Charner, which soon found 8" salvoes falling around her. The Dhonburi shifted fire back to Lamotte-Picquet after a salvo from the French cruiser put her after turret out of action. Soon she reached the safety of shallow water which the French ships could not enter for fear of grounding, but it all came too late for the hapless Thais as the Dhonburi was burning fiercely and listing heavily to starboard. At 07:50 the Lamotte-Picquet fired a final salvo of torpedoes at a range of 15,000 metres but lost sight of the Dhonburi behind an island from which she was not seen to emerge.
For the next hour the French ships patrolled the area, picking up survivors and ensuring their victory was total. At 08:40 Berenger ordered the squadron to head for home, but this coincided with the start of the expected Thai air attacks. Several bombs were dropped close to the Lamotte-Picquet but a vigorous barrage was put up by the ship's AA guns and further attacks were not pressed home. The final raid departed at 09:40, following which the victorious French squadron returned to Saigon.
The French left behind them a scene of total devastation. The Sri Ayuthia was heavily damaged and hard aground on a sand bar in the mouth of the Chantaboun river. She was later raised and repaired by the Japanese, survived the war, but was sunk by Thai shore batteries on 3rd July 1951 during an attempted revolution. The Thai transport Chang arrived at Koh Chang shortly after the French departed and took the Dhonburi in tow but she eventually capsized that afternoon and was a total loss. The torpedo boats Trad, Chonburi and Songhkli had all been sunk. The only survivors were the torpedo boat Rayong, the minelayer Nhong Sarhai and the fishery protection vessel Thiew Uthok. These three ships, which had been sheltering to the North of Koh Chang, wisely chose not to break cover and thus were not observed by the French. At a stroke the cream of the Royal Thai Navy (stand fast the four submarines) had been wiped out. The French were elated, for they had inflicted a defeat as decisive in its way as the Japanese at Tsushima. Their success is all the more notable when the difficulties of navigating and fighting in such confined waters are considered, and given the courage and tenacity which the Thai sailors exhibited during the action (a fact which the French were gracious to accept). In the end though it was all for nought - five days later the Japanese government offered to 'arbitrate' in the search for a peaceful settlement, and soon confirmed the Thai annexations. Even this state of affairs did not last for long, as Thailand was invaded later that year during the attacks on Malaya, and was forced to return her short-lived gains to France at the end of WW2.
Today the island of Koh Chang and its surrounding waters have been designated a Maritime Nature Reserve. Nothing remains visible above the surface to remind the visitor of this remarkable battle.

I had read about the naval incident, but haven't visited either the memorial or the museum - but they're definitley on my list when we're next in the area (and we WILL go back - Ko Chang is a wonderful tropical isle with beautiful beaches, waterfalls and incredible seafood - yeah.... we'll go back there!)

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